The present invention relates generally to nonevaporable getter alloys and, more particularly, to nonevaporable getter alloys formulated for general applicability without significant environmental or safety risks.
Nonevaporable getter alloys, which are sometimes referred to as "NEG alloys," are capable of reversibly sorbing hydrogen and irreversibly sorbing gases such as oxygen, water vapor, cart on oxides, and, in the case of some NEG alloys, nitrogen. Accordingly, NEG alloys ire frequently used to maintain a vacuum for thermal insulation, e.g., inside the evacuated cavities of thermos bottles, Dewar bottles, or pipes for transporting petroleum in arctic regions. NEG alloys also may be used to remove the aforementioned gases from gaseous atmospheres formed, e.g., of noble gases. For example, NEG alloys are used in lamps, particularly fluorescent lamps, to keep the atmosphere suitable for operation of the lamp. Further, NEG alloys may be used to purify inert gases by removing the aforementioned gases therefrom. In this case, the purification may be carried out either before the use of the purified gas or inside the same chamber where the purified gas is to be used, e.g., in the production of semiconductors as disclose in International Publication No. WO 96/13620 in the name of SAES Pure Gas, Inc. of San Luis Obispo, Calif., the disclosure of which is hereby incorporated by reference. In general, NEG alloys have zirconium and/or titanium as a main component and may include one or more elements selected from among the transition metals and aluminum.
NEG alloys are known in the art and the subject of several patents. U.S. Pat. No. 3,203,901 to della Porta discloses Zr--Al alloys and, in particular, an alloy having a composition of 84 wt % Zr and 16 wt % Al. This alloy is manufactured and sold by SAES Getters S.p.A. of Milan, Italy, the assignee of the present application, under the trade name St 101.RTM.. U.S. Pat. No. 4,071,335 to Barosi discloses Zr--Ni alloys and, in particular, an alloy having a composition of 75.7 wt % Zr and 24.3 wt % Ni. This alloy is manufactured and sold by SAES Getters S.p.A. of Milan, Italy, the assignee of the present application, under the trade name St 199.TM.. U.S. Pat. No. 4,306,887 to Barosi et al. discloses Zr--Fe alloys and, in particular, an alloy having a composition of 76.6 wt % Zr and 23.4 wt % Fe. This alloy is manufactured and sold by SAES Getters S.p.A. of Milan, Italy, the assignee of the present application, under the trade name St 198.TM.. Certain properties f these materials generally limit their use to some very specific applications. For example, the St 101.RTM. alloy has an activation temperature of about 900.degree. C. and therefore is used when the device in which the alloy is disposed can withstand high temperatures, whereas the St 198.TM. alloy has only a limited nitrogen sorption.
Materials similar to those disclosed in the above-described patents have even more specific applications. For example, Canadian Patent No. 1,320,336 discloses the use of the intermetallic compound ZrCo as a means for reversibly storing hydrogen because it has a high equilibrium pressure with respect to this gas and its isotopes. U.S. Pat. No. 4,668,424 to Sandrock discloses zirconium-nickel-mischmetal alloys which may include one or more other metals such as cobalt. The use of these alloys, however, is limited to the reversible sorption of hydrogen and its isotopes.
For the above-described reasons, the aforementioned alloys may be defined as alloys of specific use, and are often described and referred to in patents and technical or commercial bulletins with reference to their use in specific applications.
On the other hand, there are alloys having relatively low activation temperatures and good sorption properties for a wide variety of gases. Alloys having these functional characteristics are especially useful because they may be used in a wide range of conditions and thus in a number of different applications. These alloys may be defined as alloys of general use and will be referred to herein as such. Among the alloys of general use, the most widely used is an alloy having a composition of 70 wt % Zr, 24.6 wt % V, and 5.4 wt % Fe and an activation temperature ranging from about 350.degree. C. to about 500.degree. C., which is relatively low relative to the activation temperature of other getter materials. This alloy, which is disclosed in U.S. Pat. No. 4,312,669 to Boffito et al., is manufactured and sold by SAES Getters S.p.A. of Milan, Italy, the assignee of the present application, under the trade name St 707.TM.. The St 707.TM. alloy, however, has the drawback of containing vanadium, the compounds of which are toxic, particularly the oxides. Vanadium is contained in many alloys used for purifying inert gases, e.g., as disclosed in Japanese Laid-Open Patent Application (Kokai) Nos. 5-4809, 6-135707, and 7-242401.
One disadvantage of some known NEG alloys is that they may give rise to strongly exothermic reactions when contacted with large amounts of reactive gases, e.g., being exposed to air. Such exothermic reactions, which are more prone to occur when the alloy is heated to a temperature of at least about 200.degree. C. to about 250.degree. C., may produce temperatures above 1000.degree. C. thereby endangering the safety of nearby workers. In many applications, these NEG alloys are contained in stainless steel vessels the walls of which may melt at temperatures above 1000.degree. C. due to the formation of a eutectic co position between the NEG alloy and the vessel wall. Melting of the vessel wall n lay result in leakage of the NEG alloy at high temperature which may cause substantial damage to the surrounding environment.
NEG alloys may be inadvertently exposed to large amounts of reactive gases in a number of ways. For (example, the device containing the NEG alloy may be accidentally broken or an operator may inadvertently introduce high impurity gas, e.g., air, into the device. In the past, such events have occurred in semiconductor plants in connection with the purification of inert gases. The medium- and large-sized gas purifiers used in semiconductor plants contain large amounts of NEG alloys which are typically kept at operating temperatures of about 400.degree. C. In the event air or another reactive gas accidentally enters the purifier, such conditions promote a violent reaction between the impurities in the gas and the NEG alloy which, as mentioned above, may result in catastrophic failure of the walls of the purifier vessel and endanger those nearby the purifier. Accordingly, the use of known NEG alloys in gas purifiers presents a substantial safety risk. In addition, when the NEG alloy is the St 707.TM. alloy or one of the alloys disclosed in the aforementioned Japanese patent applications, a substantial environmental risk is presented because toxic or harmful species such as, for example, vanadium oxide, may be formed and dispersed in the environment.
In view of the foregoing, there is a need for nonevaporable getter alloys which are suitable for general use without presenting significant safety or environmental risks.